1. Field of the Invention
The present invention relates to an image forming apparatus such as a LBP (laser beam printer) or a copying machine that uses electronic photographic technology, for example, and an image formation control method.
2. Description of the Related Art
A conventional image forming apparatus is shown in FIGS. 26 and 27. FIG. 26 is a side view showing a print position adjusting mechanism in a conventional image forming apparatus. FIG. 27 is a plan view showing a part of the print position adjusting mechanism in FIG. 26. In the figures, there are shown a photosensitive drum 31, a laser device 202 that forms latent images on the photosensitive drum 31, a resist clutch (hereinafter, also referred to as xe2x80x9cthe resist rollerxe2x80x9d) 203 that determines the timing of feeding a sheet, a sheet sensor 1304 that detects a fed sheet, a deviation amount detection sensor 1305 that detects the amount of deviation of a side edge in a direction perpendicular to the direction in which a sheet is fed (hereinafter referred to as xe2x80x9cthe sheet feeding directionxe2x80x9d), an output sheet 1401, and a sheet feeding direction 1403.
In the conventional print position adjusting mechanism having the above described configuration, a CPU, not shown, obtains the deviation amount of the side edge of the sheet detected by the deviation amount detection sensor 1305 and a sheet position in the sheet feeding direction detected by the sheet sensor 1304, and transmits such information to an image control circuit, not shown. The image control circuit adjusts the timing of transferring image data that is to be transferred to a laser control circuit, not shown, that drives the laser device 202, according to the obtained information.
After a position of starting writing an image by the laser device 202 (position of starting laser beam irradiation) is set, skew of the sheet is determined according to at least two positions of the side edge of the sheet detected by the deviation amount detection sensor 1305, and thereafter displaying an error or the like is performed (for example, Japanese Laid-Open Patent Publication No. 9-219776).
However, the above described conventional print position adjusting mechanism of the image forming apparatus has a problem given below, and an improvement has been desired. That is, when sheet feeding is carried out at a high speed in order to increase printing speed, the reading capability of the sheet sensor particularly in the sheet feeding direction is insufficient; when a cheap mechanical sensor is used as a sheet sensor, the reading error is large, resulting in a deviation of the image position which is as large as the reading error.
Further, in the case that a single transparent or reflection type of optical sensor is used as a sensor that detects the leading edge of the sheet in the sheet feeding direction, when a sheet is fed with a skew, the leading edge may not be detected accurately. Therefore, it is very difficult to accurately correct the deviation of timing of feeding a sheet by a resist roller, using a single optical sensor.
Still further, the detection of the skew of a sheet is determined, according to the position of the side edge of the sheet detected by the deviation amount detection sensor 1305 during feeding of the sheet. Accordingly, image formation has already been carried out at the time a skew is detected. As a result, there is no other way but notifying the operator of the skew as an image output error.
It is a first object of the present invention to provide an image forming apparatus and an image formation control method which are capable of accurately detecting the position of a sheet in the sheet feeding direction to thereby obtain an accurate printing position even during high speed sheet feeding without an increase in the cost.
It is a second object of the present invention to provide an image forming apparatus and an image formation control method which are capable of accurately detecting skew of a sheet before formation of an image on the sheet to thereby prevent a sheet with an image formed with a low print quality due to skew of the sheet from being output.
To attain the first object, a first aspect of the present invention provides an image forming apparatus comprising an image forming section that forms an image of an original on a sheet, a resist roller that conveys the sheet to the image forming section in predetermined timing, a sheet reading section having a plurality of read pixels for reading an image of the sheet and disposed in an area which the sheet passes, between the image forming section and the resist roller in such a fashion that the plurality of read pixels are arranged in a direction perpendicular to a direction in which the sheet is fed, a leading edge detecting section that detects a leading edge of the sheet by repeatedly reading the plurality of read pixels with a predetermined period, and a start time determining section that determines timing for starting image formation by the image forming section, based on the leading edge of the sheet detected by the leading edge detecting section.
With the arrangement according to the first aspect of the present invention, the sheet reading section having a plurality of pixels is disposed between the image forming section and the resist roller in the direction perpendicular (hereinafter referred to as xe2x80x9cthe width directionxe2x80x9d) to the sheet feeding direction to detect the leading edge of the sheet. As a result, the position of the sheet in the sheet feeding direction can be accurately detected even during feeding of the sheet at a high speed, which can achieve image formation based on an accurate print position.
To attain the first object, a second aspect of the present invention provides an image forming apparatus comprising an image forming section that forms an image of an original on a sheet, a resist roller that conveys the sheet to the image forming section in predetermined timing, a sheet reading section having a plurality of read pixels for reading the image of the sheet and disposed in an area which the sheet passes, in a such a fashion that the plurality of read pixels are arranged in a direction perpendicular to a direction in which the sheet is fed, a leading edge detecting section that detects a leading edge of the sheet by repeatedly reading the plurality of read pixels with a predetermined period, a start time determining section that is responsive to the leading edge of the sheet being detected by the leading edge detecting section, for determining timing for starting image formation in the direction in which the sheet is fed by the image forming section, a side edge detecting section that detects a side edge of the sheet by repeatedly reading the plurality of read pixels read by the leading edge detecting section, and a writing start position adjusting section that adjusts a writing start position in which the image of the original is written by the image forming section in the direction perpendicular to the direction in which the sheet is fed, based on the side edge of the sheet detected by the side edge detecting section.
With the arrangement according to the second aspect of the present invention, since the leading edge and side edge of the sheet are detected by the sheet reading section having a plurality of pixels arranged in the width direction, the sheet positions in the sheet feeding direction and the width direction can be accurately detected, to thereby enable realizing image formation based on accurate print positions. Further, no special leading edge detection sensor is required, which makes it possible to decrease the number of components and hence make an image forming apparatus more compact in size.
Preferably, the leading edge detecting section repeatedly reads a part of the plurality of read pixels.
Since the leading edge detecting section thus uses a part of the plurality of pixels of the sheet reading section, the reading period is shorter and hence the pixel data density in the sheet feeding direction is higher, compared with the case of reading all of the plurality of pixels with a single read clock, resulting in improved detection accuracy.
Preferably, the side edge detecting section repeatedly reads the plurality of read pixel with a period longer than the predetermined period.
Since the side edge detecting section thus repeatedly reads pixels with a longer period than the above predetermined period, side edge detection and leading edge detection can be each carried out with the optimum detection period, thus enabling each detection to be completed in the minimum time. If the detection time becomes shorter, the distance between the resist roller and the image forming section can be shortened so that the apparatus can be made more compact in size.
Preferably, the detection of the side edge of the sheet by the side edge detecting section is carried out after the detection of the leading edge of the sheet by the leading edge detecting section.
By thus carrying out the detection of the side edge after detections of the leading edge and skew, it is possible to employ different methods for the respective detections and thus employ detection methods suitable for the respective detections to thereby increase the detection accuracy.
Preferably, the sheet reading section has a read width not less than xc2xd of the difference between the maximum sheet width that can be used with said image forming section and the maximum sheet width that can be used with said image forming section.
The sheet reading section has a reading width equal to or greater than xc2xd of the difference between the maximum sheet width that can be used with said image forming section and the maximum sheet width, the maximum and minimum sheet widths being detectable by the leading edge detecting section and the side edge detecting section, and thus the sheet positions in the sheet feeding direction and the width direction can be accurately detected for any size of sheet used in the image forming apparatus.
To attain the second object, the image forming apparatus further comprises a leading edge skew amount detecting section that detects an amount of skew of the leading edge of the sheet, based on data representive of the leading edge of the sheet detected by the leading edge detecting section.
Since the amount of skew of the sheet is thus detected based on the data that represents the leading edge of the sheet read by the plurality of read pixels by the leading edge detecting section, the calculation of the amount of the skew of the sheet and the detection of the leading edge position of the sheet can be carried out at the same time, reducing the processing time. Two separate sensors are not required for the leading edge detection and the skew detection, and thus the number of components can be decreased, thereby making the image forming apparatus still more compact in size. Skew can be accurately detected before an image is formed on the sheet, which prevents output of a sheet on which an image of a low quality is printed due to skew.
Preferably, the image forming apparatus further comprises a side edge skew amount detecting section that detects an amount of skew of the side edge of the sheet, based on the side edge of the sheet at at least two positions thereof detected by the side edge detecting section, and a leading edge skew amount correcting section that corrects the amount of skew of the leading edge detected by the leading edge skew amount detecting section, based on the detected amount of skew of the side edge detected by the side edge skew amount detecting section.
Since the amount of leading edge skew detected by the leading edge skew amount detecting section is thus corrected based on at least two side edge positions of the sheet detected by the side edge detecting section and the detected amount of side edge skew of the sheet, the detection accuracy of the amount of skew can be further improved.
Preferably, in the second aspect of the present invention, the image forming section comprises an original reading section that reads the image of the original, a laser device that irradiates a laser beam, an image carrier that has formed thereon a latent image of the image of the original read by the original reading section by the laser beam irradiated by the laser device, and a developing section that developes the latent image formed on the image carrier and transfers the developed latent image onto the sheet, and wherein the start timing determining section determines timing for starting irradiation by the laser beam onto the image carrier in the direction in which the sheet is fed.
More preferably, the distance between the sheet reading section and a transfer position in which the latent image is transferred onto the sheet by the developing section is at least equal to the sum of a distance around a periphery of the image carrier between a position of the image carrier in which the laser beam is irradiated by the laser device and the transfer position and a distance corresponding to a period of time from start of reading the sheet by the sheet reading section to determination of the timing for starting irradiation by the start timing determining section.
Preferably, the sheet reading section is disposed between the image forming section and the resist roller.
Preferably, the image forming section comprises an original reading section that reads the image of the original, a laser device that irradiates a laser beam, an image carrier that has formed thereon a latent image of the image of the original read by the original reading section by the laser beam irradiated by the laser device, and a developing section that developes the latent image formed on the image carrier and transfers the developed latent image onto the sheet, wherein the start timing determining section determines timing for starting irradiation by the laser beam onto the image carrier in the direction in which the sheet is fed, and wherein the writing start position adjusting section sets as the writing start position a position of the image carrier in which the laser beam starts to be irradiated by the laser device in the direction perpendicular to the direction in which the sheet is fed.
In this case, preferably, the distance between the sheet reading section and a transfer position in which the latent image is transferred onto the sheet by the developing section is at least equal to the sum of a distance around a periphery of the image carrier between a position of the image carrier in which the laser beam is irradiated by the laser device and the transfer position and a distance corresponding to a period of time from start of reading the sheet by the sheet reading section to determination of the timing for starting irradiation by the start timing determining section and setting of the writing start position by the writing start position adjusting section.
Further, to attain the first object, a third aspect of the present invention provides an image formation control method for an image forming apparatus comprising an image forming section that forms an image of an original on a sheet, a resist roller that conveys the sheet to the image forming section in predetermined timing, a sheet reading section having a plurality of read pixels for reading an image of the sheet, the method comprising the steps of disposing the sheet reading section in an area which the sheet passes, between the image forming section and the resist roller in such a fashion that the plurality of read pixels are arranged in a direction perpendicular to a direction in which the sheet is fed, detecting a leading edge of the sheet by repeatedly reading the plurality of read pixels with a predetermined period, and determining timing for starting image formation by the image forming section, based on the leading edge of the sheet detected in the leading edge detecting step.
To attain the first object, a fourth aspect of the present invention provides an image formation control method for an image forming apparatus comprising an image forming section that forms an image of an original on a sheet, a resist roller that conveys the sheet to the image forming section in predetermined timing, a sheet reading section having a plurality of read pixels for reading an image of the sheet, the method comprising the steps of disposing the sheet reading section in an area which the sheet passes, between the image forming section and the resist roller in such a fashion that the plurality of read pixels are arranged in a direction perpendicular to a direction in which the sheet is fed, detecting a leading edge of the sheet by repeatedly reading the plurality of read pixels with a predetermined period, determining timing for starting image formation by the image forming section, based on the leading edge of the sheet detected in the leading edge detecting step, detecting a side edge of the sheet by repeatedly reading the plurality of the read pixels read in the leading edge detecting step, and adjusting a start position of writing the image of the original in the direction perpendicular to the direction in which the sheet is fed, based on the detected side edge of the sheet.
Although the sheet reading section, the leading edge detecting section, and the side edge detecting section employ a CIS in an embodiment described later, this is not limitative, but any other configuration that can implement these functions is applicable.
Further, the writing start position adjusting section, the side edge skew amount detecting section, the leading edge skew amount correcting section, and the start time determining section, which will be described in the embodiment described later, are not limitative, but any other configuration that can implement these functions is applicable.
The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings.